1. Field of the Invention
This invention relates to a hermetically sealed autotensioner for adjusting tension of engine belts.
2. Description of Prior Arts
A conventional device related to this invention is, for example, disclosed in Japanese Utility Model Laid-open Publication No. 87652 of 1981 (Jitsu-kai-sho 56-87652).
This prior art shows a belt tensioner as mentioned in FIG. 5. A stationary cylinder 21 ha a movable cylinder 22, a movable piston 23 and a plunger 24 fitted therein, which divide the space therein into a first oil chamber A, a second oil chamber B and an air chamber C. When the temperature of the engine changes from ordinary to high or vice versa, the tension on a timing belt 25 increases or decreases, thereby correspondingly increasing or decreasing the load on the movable cylinder 22. The resulting outflow or inflow of oil from the first oil chamber A and second oil chamber B permits keeping the tension of the timing belt at a proper level.
The numeral 26 designates a return spring, while 27 and 28 denote pulleys attached to the engine. However, by lacking such means as to exactly guide the compression of the return spring 26 in the first oil chamber A, the plunger 24 in the above mechanism tends not to operate precisely.
FIG. 4 shows another autotensioner according to Japanese Utility Model Application No. 149824 of 1987 which is an improvement over the device described above. The hermetically sealed autotensioner of FIG. 4 comprises a cylinder 31, with a plunger 30 with a monobloc rod 30a and a free piston 32, which divide the space therein into a first oil chamber A, a second oil chamber B and an air chamber C, fitted therein. A cylinder guide 35 at the bottom of the cylinder 31 urges and fastens a return spring 34 compressed in the first oil chamber A.
In the above autotensioner, the tension of the belt puts a load on the tip of the rod 30a, while the up-down motion of the free piston 32 absorbs the tension. When a large thrust acts on the tip of the rod 30a, however, the rod 30a is apt to tilt because no bearing is provided at the top thereof. The slightest tilt of the rod can produce an abrasive wear on the sliding surfaces of the plunger 30 and cylinder 31 that determine the performance characteristic of the autotensioner. The wear is likely to bring about a change in the performance characteristic of the autotensioner or a rocking phenomenon.